The present invention has for its object a balance wheel for a timepiece movement provided with an adjustment device of its inertia, permitting adjusting its frequency of oscillation.
Most of the balance wheels of this type comprise adjustment screws radially screwed into the rim of the balance wheel from the outside of the latter. Swiss patent 196,706 discloses such a balance wheel. These balance wheels with radial adjustment screws have drawbacks which are principally the difficulty of guaranteeing an adjustment established in the course of operation of the movement, one or the other of the screws being likely to unscrew partially or totally; the difficulty of access to the screws to carry out adjustment of the inertia of the balance wheel, and the large friction of the perimeter of the balance wheel with air, the screws giving rise to turbulence and friction which impede the good operation and regularity of the balance wheel.
There is also known from British patent 845,773 balance wheels having in their lower surface adjustment screws whose mass is eccentric relative to their axis of rotation. If the adjustment of the balance wheel is facilitated by the accessibility of the screws from one of the surfaces instead of from its periphery, the screws extend beyond the rim of the balance wheel, giving rise to turbulence and braking, impeding the good operation of this balance wheel. Moreover, it is also difficult to ensure the desired positioning of the screws over the course of time such that misadjustment of the balance wheel is not excluded.
From Swiss 280,067, there is known a balance wheel whose rim is provided with resources opening on its upper surface, a pin being sunk in the rim in the middle of each recess. The balancing masses are engaged forcibly on these pins and permit following their angular orientation so as to modify the inertia of the balance wheel. Here again, the recesses opening laterally of the rim and the balancing masses opening outside this latter give rise to aerodynamic disturbances impeding the operation of the balance wheel.
Moreover, the machining of the recesses and the sinking of the pins into the rim are delicate and difficult operations.
Finally, there is known from Swiss 125,520, a balance wheel provided with adjustment screws screwed into the upper surface of the rim. Rondelles can be disposed between the rim and head of one or several screws to ensure adjustment. In this case, it is necessary to provide screw threading within the rim and the adjustment can take place only stepwise, by the addition of one or several rondelles, which is not sufficiently precise.
The present invention has for its object the provision of a balance wheel provided with a precise and continuous adjustment device, easy both to produce and to adjust and giving rise to only a minimum of aerodynamic disturbance during oscillating movement of the balance wheel.
The present invention has for its object a balance wheel particularly for movements of timepieces provided with an adjustment device for its inertia to adjust its frequency, which tends to achieve the objects set forth above and to avoid the drawbacks enumerated above, of the existing devices. The balance wheel according to the invention is distinguished by the characteristics set forth in claim 1.